Geometrical Optics for AO Claire Max UC Santa Cruz CfAO 2009 Summer School

Transcription

1 Geometrical Optics for AO Claire Max UC Santa Cruz CfAO 2009 Summer School Page 1

2 Some tools for active learning In-class conceptual questions will aim to engage you in more active learning and provide me with feedback on whether concepts are clear I will pose a short conceptual question (no calculations) I will ask you to first formulate your own answer, then discuss your answer with two other students, finally to report your consensus answer to the class Some web-sites & books about teaching and learning: How People Learn, Bransford, Brown, and Cocking, Editors; National Research Council, National Academy Press Page 2

3 Goals of this lecture Review of Geometrical Optics Understand the tools used for optical design of AO systems Understand what wavefront aberrations look like, and how to describe them Characterization of the aberrations caused by turbulence in the Earth s atmosphere What the optics of a simple AO system look like Page 3

4 Keck AO system optical layout: why does it look like this?? Page 4

5 Simplest schematic of an AO system PUPIL BEAMSPLITTER WAVEFRONT SENSOR COLLIMATING LENS OR MIRROR FOCUSING LENS OR MIRROR Optical elements are portrayed as transmitting, for simplicity: they may be lenses or mirrors Page 5

6 What optics concepts are needed for AO? Design of AO system itself: What determines the size and position of the deformable mirror? Of the wavefront sensor? What does it mean to say that the deformable mirror is conjugate to the telescope pupil? How do you fit an AO system onto a modest-sized optical bench, if it s supposed to correct an 8-10m primary mirror? What are optical aberrations? How are aberrations induced by atmosphere related to those seen in lab? Page 6

7 Levels of models in optics Geometric optics - rays, reflection, refraction Physical optics (Fourier optics) - diffraction, scalar waves Electromagnetics - vector waves, polarization Quantum optics - photons, interaction with matter, lasers Page 7

8 Review of geometrical optics: lenses, mirrors, and imaging Rays and wavefronts Laws of refraction and reflection Imaging Pinhole camera Lenses Mirrors Resolution and depth of field Note: Adapted in part from material created by MIT faculty member Prof. George Barbastathis, Reproduced under MIT s OpenCourseWare policies, George Barbastathis. Page 8

9 Rays and wavefronts Page 9

10 Rays and wavefronts In homogeneous media, light propagates in straight lines Page 10

11 Spherical waves and plane waves Page 11

12 Refraction at a surface: Snell s s Law Medium 1, index of refraction n Medium 2, index of refraction n Snell s law: nsin! = n" sin!" Page 12

13 Reflection at a surface Angle of incidence equals angle of reflection Page 13

14 Huygens Principle Every point in a wavefront acts as a little secondary light source, and emits a spherical wave The propagating wavefront is the result of superposing all these little spherical waves Destructive interference in all but the direction of propagation Page 14

15 Why are imaging systems needed? Every point in the object scatters an incident light into a spherical wave The spherical waves from all the points on the object s surface get mixed together as they propagate toward you An imaging system reassigns (focuses) all the rays from a single point on the object onto another point in space (the focal point ), so you can distinguish details of the object. Page 15

16 Pinhole camera is simplest imaging instrument Opaque screen with a pinhole blocks all but one ray per object point from reaching the image space. An image is formed (upside down). Good news. BUT most of the light is wasted (it is stopped by the opaque sheet) Also, diffraction of light as it passes through the small pinhole produces artifacts in the image. Page 16

17 Imaging with lenses: doesn t t throw away as much light as pinhole camera Collects all rays that pass through solidangle of lens Page 17

18 Paraxial approximation or first order optics or Gaussian optics Angle of rays with respect to optical axis is small First-order Taylor expansions: sin ε tan ε ε, cos ε 1, (1 + ε) 1/2 1 + ε / 2 Page 18

19 Thin lenses, part 1 D = lens diam. Definition: f-number f / # f / D Page 19

20 Thin lenses, part 2 D = lens diam. Page 20

21 Page 21

22 Ray-tracing with a thin lens Image point (focus) is located at intersection of ALL rays passing through the lens from the corresponding object point Easiest way to see this: trace rays passing through the two foci, and through the center of the lens (the chief ray ) and the edges of the lens Page 22

23 Refraction and the Lens-users Equation Any ray that goes through the focal point on its way to the lens will come out parallel to the optical axis. (ray 1) f f ray 1 Credit: J. Holmes, Christian Brothers Univ. Page 23

24 Refraction and the Lens-users Equation Any ray that goes through the focal point on its way from the lens, must go into the lens parallel to the optical axis. (ray 2) f f ray 1 ray 2 Page 24

25 Refraction and the Lens-users Equation Any ray that goes through the center of the lens must go essentially undeflected. (ray 3) object f f image ray 1 ray 3 ray 2 Page 25

26 Refraction and the Lens-users Equation Note that a real image is formed. Note that the image is up-side-down. object f f image ray 1 ray 3 ray 2 Page 26

27 Refraction and the Lens-users Equation By looking at ray 3 alone, we can see by similar triangles that M = h /h = -s /s object h s s f f image h <0 ray 3 Example: f = 10 cm; s = 40 cm; s = 13.3 cm: M = -13.3/40 = Note h is up-side-down and so is < 0 Page 27

28 Definition: Field of view (FOV) of an imaging system Angle that the chief ray from an object can subtend, given the pupil (entrance aperture) of the imaging system Recall that the chief ray propagates through the lens un-deviated Page 28

29 Optical invariant ( = Lagrange invariant) y 1! 1 = y 2! 2 Page 29

30 Lagrange invariant has important consequences for AO on large telescopes From Don Gavel Page 30

31 Refracting telescope 1 f obj = 1 s s 1! 1 s 1 since s 0 " # so s 1! f obj Main point of telescope: to gather more light than eye. Secondarily, to magnify image of the object Magnifying power M tot = - f Objective / f Eyepiece so for high magnification, make f Objective as large as possible (long tube) and make f Eyepiece as short as possible Page 31

32 Lick Observatory s s 36 Refractor: one long telescope! Page 32

33 Concept Question Give an intuitive explanation for why the magnifying power of a refracting telescope is M tot = - f Objective / f Eyepiece Make sketches to illustrate your reasoning Page 33

34 Imaging with mirrors: spherical and parabolic mirrors f = R/2 Spherical surface: in paraxial approx, focuses incoming parallel rays to (approx) a point Parabolic surface: perfect focusing for parallel rays (e.g. satellite dish, radio telescope) Page 34

35 Problems with spherical mirrors Optical aberrations (mostly spherical aberration and coma) Especially if f-number is small ( fast focal ratio, short telescope, big angles) Page 35

36 Focal length of mirrors Focal length of spherical mirror is f sp = R/2 Convention: f is positive if it is to the left of the mirror f Near the optical axis, parabola and sphere are very similar, so that f par = R/2 as well. Page 36

37 Page 37

38 Parabolic mirror: focus in 3D Page 38

39 Mirror equations Imaging condition for spherical mirror 1 s s 1 =! 2 R Focal length f =! R 2 Magnifications M transverse =! s 0 s 1 M angle =! s 1 s 0 Page 39

40 Cassegrain reflecting telescope Parabolic primary mirror Hyperbolic secondary mirror Focus Hyperbolic secondary mirror: 1) reduces off-axis aberrations, 2) shortens physical length of telescope. Can build mirrors with much shorter focal lengths than lenses. Example: 10-meter primary mirrors of Keck Telescopes have focal lengths of 17.5 meters (f/1.75). About same as Lick 36 refractor. Page 40

41 Heuristic derivation of the diffraction limit Courtesy of Don Gavel Page 41

42 Angular resolution and depth of field "# $ % D Diameter D "z = 8 $f 2 # D 2 Diffractive calculation light doesn t focus at a point. Beam Waist has angular width λ / D, and length Δz (depth of field) Page 42

43 Time for a short break Please get up and move around! Page 43

44 Aberrations In optical systems In atmosphere Description in terms of Zernike polynomials Based on slides by Brian Bauman, LLNL and UCSC, and Gary Chanan, UCI Page 44

45 Third order aberrations sin θ terms in Snell s law can be expanded in power series n sin θ = n sin θ n ( θ - θ 3 /3! + θ 5 /5! + ) = n ( θ - θ 3 /3! + θ 5 /5! + ) Paraxial ray approximation: keep only θ terms (first order optics; rays propagate nearly along optical axis) Piston, tilt, defocus Third order aberrations: result from adding θ 3 terms Spherical aberration, coma, astigmatism,... Page 45

46 Different ways to illustrate optical aberrations Side view of a fan of rays (No aberrations) Spot diagram : Image at different focus positions Shows spots where rays would strike a detector Page 46

47 Spherical aberration Rays from a spherically aberrated wavefront focus at different planes Through-focus spot diagram for spherical aberration Page 47

48 Hubble Space Telescope suffered from Spherical Aberration In a Cassegrain telescope, the hyperboloid of the primary mirror must match the specs of the secondary mirror. For HST they didn t match. Page 48

49 HST Point Spread Function (image of a point source) Page 49

50 Spherical aberration as the mother of all other aberrations Coma and astigmatism can be thought of as the aberrations from a de-centered bundle of spherically aberrated rays Ray bundle on axis shows spherical aberration only Ray bundle slightly de-centered shows coma Ray bundle more de-centered shows astigmatism All generated from subsets of a larger centered bundle of spherically aberrated rays (diagrams follow) Page 50

51 Spherical aberration as the mother of coma Big bundle of spherically aberrated rays De-centered subset of rays produces coma Page 51

52 Coma Comet -shaped spot Chief ray is at apex of coma pattern Centroid is shifted from chief ray! Centroid shifts with change in focus! Wavefront Page 52

53 Coma Note that centroid shifts: Rays from a comatic wavefront Through-focus spot diagram for coma Page 53

54 Spherical aberration as the mother of astigmatism Big bundle of spherically aberrated rays More-decentered subset of rays produces astigmatism Page 54

55 Astigmatism Top view of rays Through-focus spot diagram for astigmatism Side view of rays Page 55

56 Wavefront for astigmatism Page 56

57 Different view of astigmatism Page 57

58 Where does astigmatism come from? From Ian McLean, UCLA Page 58

59 Concept Question How do you suppose eyeglasses correct for astigmatism? Page 59

60 Off-axis object is equivalent to having a de-centered ray bundle Spherical surface New optical axis Ray bundle from an off-axis object. How to view this as a de-centered ray bundle? For any field angle there will be an optical axis, which is to the surface of the optic and // to the incoming ray bundle. The bundle is de-centered wrt this axis. Page 60

61 Zernike Polynomials Convenient basis set for expressing wavefront aberrations over a circular pupil Zernike polynomials are orthogonal to each other A few different ways to normalize always check definitions! Page 61

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63 From G. Chanan Piston Tip-tilt

64 Astigmatism (3rd order) Defocus

65 Trefoil Coma

66 Ashtray Spherical Astigmatism (5th order)

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68 Units: Radians of phase / (D / r 0 ) 5/6 Tip-tilt is single biggest contributor Focus, astigmatism, coma also big High-order terms go on and on. Reference: Noll

69 Seidel polynomials vs. Zernike polynomials Seidel polynomials also describe aberrations At first glance, Seidel and Zernike aberrations look very similar Zernike aberrations are an orthogonal set of functions used to decompose a given wavefront at a given field point into its components Zernike modes add to the Seidel aberrations the correct amount of low-order modes to minimize rms wavefront error Seidel aberrations are used in optical design to predict the aberrations in a design and how they will vary over the system s field of view The Seidel aberrations have an analytic field-dependence that is proportional to some power of field angle Page 69

70 References for Zernike Polynomials Pivotal Paper: Noll, R. J. 1976, Zernike polynomials and atmospheric turbulence, JOSA 66, page 207 Books: e.g. Hardy, Adaptive Optics, pages Page 70

71 Considerations in the optical design of AO systems: pupil relays Pupil Pupil Pupil Deformable mirror and Shack-Hartmann lenslet array should be optically conjugate to the telescope pupil. What does this mean? Page 71

72 Define some terms Optically conjugate = image of... optical axis object space image space Aperture stop = the aperture that limits the bundle of rays accepted by the optical system symbol for aperture stop Pupil = image of aperture stop Page 72

73 So now we can translate: The deformable mirror should be optically conjugate to the telescope pupil means The surface of the deformable mirror is an image of the telescope pupil where The pupil is an image of the aperture stop In practice, the pupil is usually the primary mirror of the telescope Page 73

74 Considerations in the optical design of AO systems: pupil relays Pupil Pupil Pupil PRIMARY MIRROR Page 74

75 Typical optical design of AO system telescope primary mirror Deformable mirror Pair of matched offaxis parabola mirrors collimated Science camera Beamsplitter Wavefront sensor (plus optics) Page 75

76 More about off-axis parabolas Circular cut-out of a parabola, off optical axis Frequently used in matched pairs (each cancels out the off-axis aberrations of the other) to first collimate light and then refocus it SORL Page 76

77 Concept Question: what elementary optical calculations would you have to do, to lay out this AO system? (Assume you know telescope parameters, DM size) telescope primary mirror Deformable mirror Pair of matched offaxis parabola mirrors collimated Science camera Beamsplitter Wavefront sensor (plus optics) Page 77

78 Review of important points Both lenses and mirrors can focus and collimate light Equations for system focal lengths, magnifications are quite similar for lenses and for mirrors Telescopes are combinations of two or more optical elements Main function: to gather lots of light Aberrations occur both due to your local instrument s optics and to the atmosphere Can describe both with Zernike polynomials Location of pupils is important to AO system design Page 78